The present invention relates to a control system for four-wheel drive vehicles, and more particularly to a system for controlling the torque capacity of a transfer clutch for four-wheel drive vehicles during braking of the vehicle.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission train can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. In such a vehicle, when the vehicle negotiates sharp corners, braking phenomenon called "tight corner braking" occurs. In order to eliminate the braking phenomenon, a four-wheel drive vehicle, in which a four-wheel drive power transmission train is converted to a two-wheel drive power transmission train at a sharp corner has been proposed. However, although such a system prevents the tight corner braking at sharp corners by the two-wheel drive vehicle system, characteristics of the four-wheel drive vehicle such as its driveability on slippery roads, uphill driving and other conditions are not effected.
In order to prevent the tight corner braking with while keeping a four-wheel drive power train, it is preferable to employ a friction clutch as a transfer clutch and to control the torque capacity of the transfer clutch so as to let the clutch slip in accordance with driving conditions of the vehicle.
On the other hand, it is preferable to establish the four-wheel drive at the braking of the vehicle. Japanese Utility Model Laid Open No. 59-53128 discloses a four-wheel drive vehicle having a transfer clutch, in which the transfer clutch is engaged to provide the four-wheel drive, when the depression force exerted on a brake pedal of a vehicle exceeds a predetermined value. However, the system has a disadvantage that the four-wheel drive is not established when the depression force is small.
Considering the braking effect in a four-wheel drive vehicle having a friction transfer clutch, when the vehicle is suddenly braked, the auxiliary wheels tend to be locked with slipping of the transfer clutch if the torque capacity of it is set to a small value. The locked auxiliary wheels skid, which will decrease the braking effect of the vehicle. In order to eliminate such a disadvantage, it is preferable to increase the torque capacity of the transfer clutch so as to transmit the torque of the engine to the auxiliary wheels when the depression force on the brake pedal is large.
To the contrary, when the vehicle turns sharp corners, the depression force may be small. Accordingly, at a small depression force, it is desirable to decrease the torque capacity in order to prevent the tight corner braking.